The present invention generally concerns a supercharged internal combustion engine and more particularly to a supercharged internal combustion engine having a pair of turbochargers each connected between separate inlet and outlet ducts which can be selectively closed to improve the performance of the engine.
An internal combustion engine capable of supercharging disclosed in JP-A 61-210224 provides for the admission to the two exhaust gas turbochargers to be always equal over the whole load and rotational speed range. In consequence, only relatively low exhaust gas flow velocities are available in the lower rotational speed ranges. The supercharge rotational speeds are therefore so low that there is only a very sluggish build-up of supercharge pressure in the case of a positive change of load. The flow velocities in these operating ranges are also low on the inlet side of the supercharger so that, because of the reduced turbulence in the combustion space of the internal combustion engine, satisfactory mixture formation is not ensured, particularly in the case of diesel engines.
An internal combustion engine which is supercharged by two exhaust gas turbochargers, of which one can be put out of operation, if required, by means of shut-off elements in the exhaust and inlet regions, is disclosed from German Patent (DE-PS) 850 965. The flows in the inlet and exhaust ducts of the two turbochargers are connected together so that the two supercharged devices act like a single turbocharger when a second turbocharger is added. However, German Patent (DE-PS) 850 965 gives no information on how the individual inlet and exhaust ducts are to be arranged in the case where a four-valve cylinder head is used.
In JP-A 60-79123, it is disclosed to arrange the valves on a four-valve cylinder head in such a way that with respect to a hypothetical square, whose corners are formed by the two inlet and exhaust valves, the two inlet valves lie on one diagonal and the two exhaust valves lie on the other diagonal.
An object of the present invention is to produce a supercharged internal combustion engine having a pair of exhaust gas turbochargers each connected between separate inlet and outlet ducts which are selectively closed to provide a clearly improved build-up of supercharge pressure in the case of a positive change in load.
It is a further object of the present invention to provide a supercharged internal combustion engine wherein optimum mixture formation in the combustion space can be achieved even in the lower rotational speed ranges.
The invention achieves these and other objects by providing a shut off element at each of an inlet side and an exhaust side connected to the same turbocharger which can be selectively closed to improve engine performance.
Because, in the internal combustion engine according to certain preferred embodiments of the present invention, one of the two exhaust gas turbochargers can be separated from both the turbine side and the compressor side by suitable shut-off elements, it is possible for the whole of the exhaust gas of a cylinder to be pumped to the turbine via one exhaust duct only. In consequence, the exhaust gas achieves a relatively high flow velocity. The supercharger rotational speed can therefore be held at a relatively high level even in the lowest rotational speed ranges when one exhaust gas turbocharger is separated from the system so that a rapid build-up of supercharge pressure is ensured in the case of a positive change in load.
In accordance with further preferred embodiments of the present invention, as on the exhaust side, the whole of the induced gases flow into the combustion space via only one inlet duct instead of two when one exhaust gas turbocharger is separated from the system. This, of course, again leads to a higher flow velocity so that optimum conditions for intensive mixing of the fuel with the combustion air are present even in the lower rotational speed ranges.
The mixture formation is improved still further according to further preferred embodiments of the invention if the inlet duct on the non-closable inlet side is, in addition, designed as a swirl duct. By this means, optimum adaptation of the swirl level necessary in the cylinder is provided for good mixture formation over various load and rotational speed conditions of the internal combustion engine by appropriately switching on or off one exhaust gas turbocharger. If the non-closable inlet duct is designed with, for example, high swirl, the swirl in the cylinder can be held substantially constant over the whole rotational speed range of the internal combustion engine by the addition, if appropriate, of a weak-swirl second inlet duct. On the other hand, however, high turbulence levels can be generated in high rotational speed ranges in the case of spark-ignition engines, for example.
The diagonal arrangement of the valves in accordance with the teachings of the present invention is advantageous in that it ensures an optimum heat distribution in the cylinder head, in contrast to the arrangement with inlet and exhaust valves parallel to the engine axis, in which the temperature drop between the hot exhaust side and the cold (relative to the exhaust side) inlet side is very large.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.